1. Field of the Invention
The present invention relates generally to focusing servo apparatus and, more particularly to a focusing servo apparatus for use with an optical disc apparatus or the like.
2. Description of the Prior Art
When an information is recorded on and reproduced from an optical disc, a light (laser beam) must be controlled so as to be properly focused on the optical disc. When the focusing control is actuated, a sweep signal having a sawtooth or triangular waveform shown in FIG. 1, for example, is supplied to a focusing actuator (not shown), whereby an objective lens is moved in the direction close to the optical disc and is also moved in the direction distant from the optical disc after having reached to a predetermined position. In the process during which the objective lens approaches to or goes away from the optical disc as described above, a focusing error signal shown in FIG. 2, for example, is generated. When a focus servo loop is turned on at timing near the timing at which the focusing error signal is just reduced to zero, then the focus servo loop is locked-in, thereby the focusing servo being executed.
When the triangular or sawtooth wave signal is supplied to the focusing actuator as the sweep signal, there is then the problem that the objective lens cannot always be moved smoothly. An apparatus, which is required to have an earthquake-proof property, for example, employs a focusing actuator of a shaft-slide type configuration. In the focusing actuator of shaft-slide type configuration, a shaft is provided in the direction (focusing direction) perpendicular to a record medium and an objective lens is slid along this shaft as a guide. Also, in order to make this focusing actuator as an earthquake-proof type, the objective lens (i.e., objective lens holder) is urged against the shaft so that the objective lens holder cannot be moved in the direction perpendicular to the shaft so easily, thus resulting in a friction force between the objective lens and the shaft being increased.
Accordingly, when the objective lens is moved in response to the sweep signal, then a force larger than a statical friction force is needed when the objective lens starts being moved from the stationary state. However, after the objective lens starts being moved, such friction becomes a dynamical friction, so that the friction force is lowered rapidly. As a consequence, when the objective lens is moved by the triangular wave or sawtooth wave sweep signal, then the objective lens is rapidly moved from the stationary state so that a time T shown in FIG. 2 is reduced considerably (e.g., 500 microseconds). The time T assumes such a duration in which the focusing error signal rises to a positive peak from zero level, then falls through the zero level to a negative peak and returns again to the zero level from the negative peak. When this period T is reduced, even if the focusing servo loop is turned on at the timing at in which the focusing error signal crosses the zero level, then the focusing servo cannot be locked in without difficulty.
In order to remove the above drawbacks, it is proposed to supply a repetitive signal such as a sine wave signal or the like to a tracking actuator when the focusing servo is actuated. If the repetitive signal such as the sine wave signal or the like is supplied to the tracking actuator, then the objective lens is moved by a very small amount in the tracking direction so that a statical friction does not occur, thereby a smooth sweep being realized.
However, if the repetitive signal is supplied to the tracking actuator as described above, the focusing servo and the tracking servo must be effected in association with each other, which makes the servo control complex.